The present invention relates to an air induction system for an internal combustion engine, with a housing for receiving an air filter and with a supply of air from at least two induction tubes. The air aspirated by such an air induction device is fed, after passing through the air filter it contains, into the individual combustion chambers. Since in this case the aspirated air forms a more or less continuous air column between the carburetor and valves and the intake openings of the air induction device and pulsates according to the rate of the opening and closing of the valves, a loud noise is created, mainly at the intake openings. Both this noise and the noise created by aspiration through a tube or the like, are to be damped as a secondary function in addition to the filtration of the intake air.
For this purpose an air induction tube is disclosed in German Patent No. 1,714,100, which leads into an air filter and forms two technically parallel flow channels of different length which, as a special feature, are joined together over their entire length by a slot. This slot has a width of a few millimeters for noise suppression by interference. The disadvantage here lies for one thing in the fact that. due to the connecting slot, the two channels must be arranged in a bundle, and for another thing that the noise suppression is considered insufficient.
A further solution is described in European Patent No. 242 797 in which an induction noise suppressor has two diffusers which vary in their cross section and lead into the induction noise suppressor. They are of equal length, but have different cross sections, they are uncoupled acoustically from one another, in contrast to the patent mentioned above, and each diffuser contains a shut-off means which can be actuated alternately according to the load on the motor and the motor speed. The suppression of the noise is accomplished substantially by the diffusers, which are opened alternately depending on the rotational speed range. The inherent frequency for the damping is determined by the average cross section of the diffuser that is open in each case, and by the use of the second diffuser an earlier starting suppression of the induction noise can thus be achieved. This solution, however, involves a considerable amount of trouble.
It is therefore the object of the invention to provide an air induction device whose use is as flexible as possible, which requires minor modifications of common commercial air intakes, and which enables optimum suppression of the induction noise at low cost.
This object is achieved by the fact that at least two induction tubes are of different length and closing means is associated with at least one induction tube and the latter is in the form of a supplementary induction tube. The difference in the lengths of the preferably two induction tubes permits construction together with the housing as a Helmholtz resonator, which suppresses induction noise in an especially advantageous manner. By means of the one closable induction tube the Helmholtz resonator can be tuned to two frequency ranges or rotational speed ranges, preferably one range for low to medium speeds and one for high rotational speeds. In one embodiment with a permanently open induction tube and a supplementary induction tube that can by closed by a closing means, a broadening or shifting of the frequency range is effected by switching in or opening the additional induction tube. The length is preferably such that, at a low frequency, corresponding for example to a motor speed of about 1000 rpm, it is tuned so that it corresponds approximately to half of the sound wavelength belonging to this frequency. In this way an optimum suppression of the noise is accomplished by interference, since no resonance can occur. Likewise, the length can of course amount to a multiple of the half wavelength. Since at low frequencies the wavelengths of the sound are greater, it is advantageous to make the open induction tube longer than the supplementary induction tube. Advantageously, the diameter of the always open induction tube is larger than that of the at least one supplementary induction tube. The cross section of the tubes is optional within certain limits, but an approximately circular cross section is preferred.
Especially advantageous is an embodiment in which the housing is configured as a resonator suppressing with the air supply the motor speed-related noise of the air induction, is connected to the motor preferably as a series resonator, and especially forms a Helmholtz resonator whose volume is variable by the closing means and can be adjusted to the rotational speed. In this way not only can the noise produced by the air intake of the motor can be better suppressed, but also through the use of the air filter housing with the induction tubes the number of necessary components can be kept low. Any desired air filter can be contained in the housing, for example a round or rectangular paper bellows filter.
A further improvement of the noise suppression, especially in the lower and medium speed range, can be achieved with the supplementary induction tube closed if in at least one closeable supplementary induction tube the closing means is mounted at a certain distance from the housing, preferably at the end of the supplementary induction tube. In this case when the supplementary induction tube is in the closed state, it forms a short tubular projection as an extension of the housing.
Advantageously, in one position at least one supplementary induction tube is closed by the closing means, and the housing and the open induction tube together form a Helmholtz resonator for low to medium speeds of the motor, especially at about 3,000 rpm. Accordingly, the length and diameter of the induction tube as well as the volume of the housing must be tuned for this speed range. A further improvement can be made according to the invention if the closed supplementary induction tube forms a tubular projection on the housing and for these speeds it is a so-called quarter-wavelength tube, which means that the length of the tubular projection, especially up to the closing means, corresponds approximately to one-quarter or an uneven multiple of one quarter of the wavelength xcex of the sound generated. Thus a standing wave builds up in the quarter-wavelength tube and produces a suppression of noise.
In another embodiment of the invention, for high speeds of the motor, especially from about 3,500 rpm, at least one supplementary induction tube is open. If a plurality of closeable supplementary induction tubes are used, additional tubes can be opened or closed as the speed increases. Preferably the housing forms together with its volume and geometry, i.e., the length and cross section of the tubes, a Helmholtz resonator for high rotational speeds. Another advantage in this case is that with the additional induction more air can be fed to the engine for the attainment of higher power and improved efficiency.
It is true that the optimum damping occurs basically at one frequency or in a very narrow frequency range, yet the suppression is still sufficient at frequencies above it. The choice of the speed ranges is given above only by way of example and, since they can be chosen freely by changing the geometry, they can be however desired. For example, frequencies that are found to be especially obnoxious, or frequencies that should be avoided, can be given the maximum suppression. By increasing the number of closeable supplementary tubes, a refined tuning can be achieved, and with it an improved suppression of induction noises.
Advantageously, the closing means are in the form of movable flaps, which if preferably round flaps can be rotated on their central axis. A preferred means for their operation is an electrical servo motor, which can preferably be flange-mounted directly on the supplementary induction tube. The motor is operated by a control, as for example the electronic control of the internal combustion engine. Another possibility is a pneumatic operation by means of the vacuum developed in the intake tract, or a direct mechanical coupling which operates the closing means directly according to the rotational speed in the manner of a centrifugal force regulator or the like. In one preferred embodiment of the invention, the induction tubes open into the housing at a distance apart from one another, especially from different sides and/or in different directions, and they preferably can be mounted at a certain distance from the housing on tubular projections or the like formed thereon. Due to the difference in the way the tubes lead into the housing they can be independent of one another in their action in suppressing noise. The housing and/or the induction tubes can be made of a thermoplastic, as for example polypropylene or polyamide, preferably by injection molding. Since they are fastened on short tubular stems extending from the housing, they can be secure against vibration.
At least one induction tube, preferably the always open one, terminates in the area of a bumper or of an upper fastening on the body, especially behind it, and at least one supplementary induction tube terminates in the area of the wheel housing, advantageously above the wheel casing. Air intake at different points prevents interference; air intake in the area of the wheel housing by the supplementary induction tube assures a supply of cold air which is especially advantageous to a high rotational speed and therefore to the power range of the motor.
In one embodiment of the invention an air induction device is provided whose always open induction tube terminates for air intake in the motor compartment behind the bumper, and whose supplementary induction tube, which has at its end a round rotary valve operated by an electric servo motor, runs from the housing and ends in the area above the wheel casing. At low and medium speeds, especially up to about 3,000 rpm, the valve closes the supplementary induction tube, while the closed tubular projection is a quarter-wavelength tube for the suppression of noise produced by the air intake, and the housing with the open induction tube forms a Helmholtz resonator. At high speeds, especially above about 3,500 rpm, the valve is opened for the additional aspiration of air through the supplementary induction tube, while preferably the total length of the supplementary induction tube and its cross section form with the housing and the open induction tube a Helmholtz resonator for the suppression of noise at these speeds.
By providing an always open induction tube, the number and cost of closing means needed can be considerably lowered, as well as liability to malfunction. The invention is based advantageously not on noise suppression by alternation between tubes, but by the additional use of at least one supplementary induction tube.
These and additional features of preferred embodiments of the invention will be found not only in the claims but also in the description and the drawings, the individual features being applicable individually or severally in the form of subcombinations in the embodiment of the invention and in other fields and may represent advantageous as well as independently patentable embodiments, for which protection is hereby claimed.
The division of the application into individual sections and subtitles do not limit the general validity of the statements made thereunder.